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Sample sizes for prevention trials have been too small
F Ederer1, T R Church, J S Mandel
1School of Public Health, University of Minnesota, Minneapolis.
Abstract:
Planners of several large prevention trials have overestimated the expected incidence of events in the control group, largely because they failed either to recognize or to adequately correct for various effects of population selection. Consequently, the studies have been too small in size or too short in duration to achieve their stated objectives. The selection effects include those engendered by the choice of the target population, the self-selection of volunteers, and protocol exclusions. This paper presents a taxonomy of these effects and the likely direction of their influence on the incidence of events and on mortality rates from other causes. Little information is available to help sample-size planners in adjusting for these effects. A few studies have provided information on the extent to which control group incidence rates have fallen short of expectations. In particular, researchers from the University of Minnesota's Colon Cancer Control Study have provided a detailed comparison of event incidence and all-cause mortality rates with general population rates. (AM J Epidemiol 1993;137:797-810). Other studies should publish similarly detailed information to assist sample-size planners of prevention trials. Until more information is published, this paper provides preliminary guidelines for prevention trial sample-size planning.
Insights
Prevention trials often overestimate event incidence due to uncorrected population selection effects. This leads to underpowered studies, necessitating better sample-size planning for future prevention research.
Area of Science:
- Epidemiology
- Clinical Trial Design
Background:
- Large prevention trials frequently overestimate event incidence in control groups.
- This overestimation is often due to failure to account for population selection effects.
Purpose of the Study:
- To present a taxonomy of population selection effects impacting prevention trials.
- To provide guidelines for adjusting sample-size calculations in prevention studies.
Main Methods:
- Review and categorization of population selection effects (target population choice, self-selection, protocol exclusions).
- Analysis of the influence of these effects on event incidence and mortality rates.
Main Results:
- Population selection effects can lead to underpowered or shortened prevention trials.
- Limited data exists on adjusting for these effects in sample-size planning.
Conclusions:
- Accurate sample-size planning in prevention trials requires accounting for population selection effects.
- More detailed data on incidence and mortality in control groups is needed from various studies.